Bountiful Roebuck Bay

Editor's Note: Today's story is the answer to the August Puzzler.

Tides are among the clearest signs of the Moon’s pull on Earth. Just 239,000 miles (385,000 kilometers) away, the Moon's gravity pulls on Earth’s oceans and solid crust, subtly distorting them into a more oblong shape with bulges roughly extending toward and away from the Moon.

High tides happen throughout Earth's oceans, but in some places they leave an unusually strong fingerprint on the landscape. Among those places is Roebuck Bay, a crescent-shaped feature in the Kimberley region of Western Australia.

Roebuck Bay's tidal range can reach a remarkable 9 meters (30 feet). Rising and falling tides repeatedly inundate and expose expansive mudflats, flood broad mangrove forests and salt marshes, and feed branching networks of tidal drainage channels. Many parts of Australia have tidal ranges of 2 meters or less. But the large range at Roebuck Bay is mostly a consequence of northwestern Australia's unusually wide, shallow continental shelf, which helps amplify tides as they approach the coast.

The image at the top of the page shows the bay on March 18, 2026, when water levels were high. Green mangrove forests grow thickly along the shoreline and line the mouths of a network of evenly spaced, linear tidal creeks. These mangrove forests are dynamic. Analysis of decades of Landsat observations shows them expanding westward by nearly 2 meters per year as sediment from the waterways to the east accumulates in the sheltered bay.

Farther inland, branching networks of tidal drainage channels connect with the tidal creeks, giving the bay its feathered appearance. These channels, partially obscured by thick vegetation in March, were more visible earlier and later in the year, when vegetation was thinner.

Monsoonal rains, typically falling between December and March, transform the landscape around the bay into lush grasslands and seasonal wetlands. As the rains fade in May and June and the dry season takes hold, these ephemeral grasses and sedge ecosystems die back, turning the landscape shades of gold and brown. The consistent spacing of the tidal creeks is likely not a tidal effect; it appears to be influenced by the regular spacing of linear dunes in the broader region, as seen in the plains to the east.

The bay's dramatic tidal and seasonal changes are striking from above, and they also support a bounty of life on the ground. The mangroves serve as nurseries for crustaceans and fish, and the mudflats teem with dozens of types of invertebrates, including snails, worms, crabs, clams, and cockles. Shells and snails can reach an abundance of 2,500 per square meter, according to the Australian government.

This bounty of marine life is a major draw for birds. The bay, one of the most important sites for migratory birds in Australia and a key stop on the East Asian-Australasian Flyway, regularly hosts hundreds of thousands of birds, including plovers, godwits, and knots.

NASA Earth Observatory images by Lauren Dauphin, using Landsat data from the U.S. Geological Survey. Story by Adam Voiland.

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Full text from nasa.gov (public domain).